Gradient-index glass is a glass in which the index of refraction varies spatially in a controlled manner. Often, optical elements made from such glass are cylindrical; and as such, the index changes in a radial fashion with the highest index located along the axis and the lowest index located at the outer surface. The total change in the index from the axis to the outer surface is referred to as delta-n.
A graph of index of refraction versus spatial position is commonly referred to as the index profile. The manner in which the index profile changes with different wavelengths of light is called the index profile dispersion. delta-n, the shape of the index profile and the index profile dispersion are the three primary characteristics of a gradient-index glass.
The use of gradient-index glass in optical systems provides many advantages over homogeneous glass. These advantages include greater simplicity by reduction of the total number of optical elements in a system and improved performance. At the present time, however, lack of suitable materials limits the development and application of gradient-index optics. Drawbacks of currently available materials produced by ion-exchange techniques include small size, poor environmental and thermal stability, and a limited variety of optical characteristics such as index profile dispersion and base index of refraction.
Recently, a number of researchers have been pursuing a number of avenues for making gradient-index glass which utilize porous silicate preforms. See, e.g., U.S. Pat. Nos. 3,938,974 and 4,302,231. These preforms are fabricated either by leaching a phaseseparated glass or by sol-gel methods. The essential method is first to create and then fix into place a concentration gradient of refractive index modifying dopants within the porous preform. The preform then is dried and heated until it becomes a pore-free glass element with an index gradient. These techniques have been reviewed by Yamane in U.S. Patent Serial No. 820,486 . filed Jan. 16, 1986, now U.S. Pat. No. 4,686,195, issued Aug. 11, 1987.
One method for producing gradient-index glass by a diffusion process in inorganic oxide gel monoliths was first proposed by Mukherjee in 1981. See Mukherjee, S.P., Gradient Index Lens Fabrication Processes: A Review, in Topical Meeting on Gradient-Index Optical Imaging Systems, May 4-5, 1981, Honolulu, Hawaii, Optical Socity of America (1981), pages Tu A1-1 to Tu A1-5. No samples or results were presented, however. The potential advantages of using sol-gel precursors in the production of gradient-index glass include: (1) relatively large diffusion coefficients (2) low energy consumption during most of the process; and (3) the ability to introduce a broad variety of index-modifying dopants into the sol-gel preform.
Yamane (U.S. Pat. Ser. No. 820,486) produced a crude gradient-index glass by a sol-gel technique. This technique involved mixing a silicon alkoxide with water, a source of boron oxide, and an aqueous metal salt solution which is the source of modifier cations. This mixture forms a gel which then is placed in a solution to leach out some of the metal salts contained within it and to have other metal salts introduced into it by diffusion. The gel then is dried and sintered into glass.
The main problem with this technique is that since the index modifiers are introduced as salts they are not incorporated into the structural network of the gel until it is heated to a high temperature. The modifier cations are thus free to migrate during the drying step, and this causes asymmetry in the final index profile.
Shingyouchi et al., Electronics Letters, 22:99-100, 1108-1110 (1986), reported a slightly different technique than Yamane's. In this technique, germanium is the index modifying cation, but it is introduced as an alkoxide rather than as a salt. The index modifier thus is fully incorporated into the gel structure, and the index profile does not suffer from uncontrollable asymmetry.
First, tetramethoxy silane (a silicon alkoxide) is combined with tetraethoxy germanium (a germanium alkoxide), ethanol, water and hydrochloric acid. The mixture forms a gel which is placed in water to leach out some of the germanium component. The gel then is washed in methanol to fix the germanium concentration gradient, and then is dried and sintered into gradient-index glass. Shingyouchi et al. also used titanium to replace germanium as the index modifying cation. The resulting glass was a rod 2mm in diameter with a delta-n of 0.013.
The method of Shingyouchi et al. involves the use only of two components: silica and an index modifying oxide, such as germanium dioxide or titanium dioxide. The method can be generalized to substitute zirconium dioxide as well.
These binary systems, however, will yield gels which shrink considerably during drying. This large shrinkage results in a dense gel which is difficult to sinter without fracturing or bloating. The SiO.sub.2 /TiO.sub.2 binary tends to crystallize at elevated temperatures if the TiO.sub.2 content exceeds 4 to 5 mole percent. The SiO.sub.2/ZrO.sub.2 binary gels tend to bloat at elevated temperatures because the outside portions of the gel collapse before the inside portions, thereby trapping any released gasses. As a result, the method and compositions disclosed by Shingyouchi et al. suffer from several flaws.